I don't think that when money is not involved by those participants,it is called gamble. Gambling means that you must place a bet and that is what will make you more serious in it because you know that if you lose,your money is gone. In this case,there is nobody losing anything but you can win when you predict correctly,gambling is either profit or loss. Therefore,I can call such as a reward because for you to get rewarded,you must have shown some quality that is outstanding from the rest people that you took part in an event or in a assignment.
Yes, there must be something at stake to be considered gambling. It can be direct money or any other valuable physical or digital goods. wherever it goes, there must be financial gain and loss in some way, then it can be considered as gambling. however, not only casino or sports bets are considered gambling, any kind of betting is considered gambling. But there must be a financial relationship. so according to the op's topic it can be said Without money, nothing is not qualify as gambling
You are right that it is not a condition of the bet and it cannot be called a bet if it does not use money but the prize is getting money, is that considered as gambling?
Since the Op said the prediction is done for free and if the prediction is correct there will be a real money prize from here I think this might not be considered as gambling but isn't it like that would trigger people to gamble with real money?